1. Field of the Invention
The invention relates to methods of producing a polyacrylate-urethane (PAU) core-shell copolymer and, more particularly, to a method of producing super soft PAU core-shell copolymer dispersions by a solvent free process.
2. Description of Related Art
Regarding commercially available PAU core-shell copolymer dispersions, their prepolymers have a very high viscosity. Thus, 5 wt %-30 wt % of an organic solution is added in the process for decreasing viscosity. A large amount of volatile organic compounds (VOCs) is generated in the drying process. However, high VOCs may damage the environment, contribute to global warming and climate change, and are harmful to health.
In 2011 after the Detox movement held by a Greenpeace organization, Adidas, C&A, H&M, Li-Ning, Nike and PUMA have formed the zero discharge of hazardous chemicals (ZDHC) organization. ZDHC declare that at 2020 the raw materials and the manufacture process of footwear and textile industry should reach ZDHC standards with zero discharge of hazardous chemicals. Most polyurethane dispersions (PUDs) were produced by an acetone process there is a large amount of solvent involved, and some solvent residuals are left in the final products. The PUD products made by the acetone process will not reach the ZDHC standards.
For increasing softness, a softening agent, a plasticizer or the like is added to resin to improve the smoothness felt. However, these additives contain VOCs chemicals harmful to the environment. An easy technique is to increase the amount of the soft monomer content of the polyacrylate dispersion (PAD). However, such products are sticky and have a low tensile strength. Typically, the PAD is made by blending PAD and PUD. For PAU dispersions, when the added polyacrylate has more than 50 wt % of polyurethane (PU), layer separation and storage instability may occur. S. J. Son et al. chose PUD as the solution and added methyl methacrylate (MMA) to PUD. They found that the emulsion is not stable and that its particles may precipitate when the added MMA has more than 50 wt % of PUD. This is because PUD and PAD are immiscible and because chemical bonds are not formed in the mixing process.
Thus, the need for improvement still exists.